Contaminated soil sites pose serious environmental problems. Many industrial activities are likely to result in metals contamination of soil, such as mines, smelters, foundries, metal fabricating and plating industries, cement plants, shipyards, metal recycling facilities, bulk terminals, wood treatment yards, and surface finishing (sandblasting) industries. Soil contaminated with toxic heavy metals and organics often poses a long term environmental hazard to terrestrial and aquatic organisms in the vicinity of the contaminated site. Moreover, contaminated soil reduces property values and limits the real estate development or redevelopment potential of the site. Remediation of contaminated sites is desirable to reduce or eliminate these problems.
Most heavy metal contaminants are present in soil in the relatively small particle size clay, silt, humic and organic soil fractions. The larger particle size sand and gravel fractions are usually relatively uncontaminated. Several soil treatment techniques are known in the prior art which include a segregation step for concentrating contaminants by means of particle size separation. The disadvantages to the use of mechanical separators include high energy and maintenance costs and relatively poor separation of soil fractions. Often it is necessary to initially subject the soil to grinding and screening operations. Moreover, the efficiency of segregation varies depending upon the moisture content of the soil.
German patent document 4303529 is illustrative of the prior art. This reference discloses an apparatus and procedure for the chemico-physical and bio-physical treatment of contaminated soil which includes the step of subjecting the soil particles to shear forces within a water-filled washing reactor to separate the clay and organic fraction from coarser soil particles. The shear forces are generated by mechanical drums which rotate within the washing reactor to create air-water eddies.
The applicant has determined that a more efficient means for segregating soil by particle size is by injecting a stream of compressed air directly into a fluid mixing chamber. Fluidized beds and air flotation cells for carrying out chemical and biological reactions under controlled conditions are well known in the prior art, but they are not adapted for effectively segregating raw soil particles and managing the resulting solid fractions.
Another drawback of some prior art soil treatment systems is that they cannot be used to treat the contaminated soil in situ (i.e. at the site of contamination). Rather, the contaminated soil must be excavated and transported to the remediation site for treatment. The cleaned soil is then transported back to the excavation site. The need to transport large volumes of soil significantly increases the overall cost of the treatment process. The possibility of spillage of contaminated soil during transportation is also a concern.
The need has therefore arisen for a mobile soil treatment apparatus and method employing a fluidized bed reactor capable of segregating raw soil into contaminated and uncontaminated fractions in a continuous operation without resorting to mechanical grinders or separators.